1. Field of the Invention
The present invention relates to an electrical insulating film and a condenser, more specifically, it relates to an electrical insulating film which comprises a specific styrene polymer excellent in electrical insulating properties, heat resistance and the like, and further relates to a condenser which comprises said electrical insulating film and metal electrodes.
2. Description of the Related Arts
Hitherto, polyethylene terephthalate (PET), polypropylene (PP), styrene polymer having an atactic configuration and the like have been used as materials for electrical insulating films. However, these materials have insufficient heat resistance. Particularly, they have low resistance to heat of soldering during production of SMD (surface mount device) which has recently become an particularly important problem, resulting in difficulty in processing operation.
Accordingly, polyphenylene sulfide (PPS), polyimide and the like have been developed as a heat resistant material. However, these materials are not practical because they are expensive and have problem in insulating properties.
The film comprising styrene polymer having a syndiotactic configuration, which the present inventors have previously proposed (see Japanese Patent Application Laid-Open No. 316246/1989) is excellent in heat resistance, chemical resistance, electrical properties, mechanical strength and the like, and expected to be used as an electrical insulating material instead of the above materials.
However, the previously proposed film which comprises a styrene polymer having a syndiotactic configuration has various problems for practical use, for example, when it is formed into a thin film, it may fail to show sufficient dielectric strength, and dielectric dissipation factor particularly in high frequency region may be high.
Accordingly, the present inventors have studied intensively to develop a film comprising styrene polymer having a syndiotactic configuration which can be practically used as an electrical insulating material. As the result, it has been found that a film of the styrene polymer, wherein impurities mixed during an operation for production thereof is controlled at low concentration, is adequate to the above purpose and suitable an as electrical insulating film. Further, it has been found that the above film having a certain thickness and crystallinity provided with metal electrodes is useful as a condenser. The present invention has been accomplished based on such findings.